The present invention relates to a method for obtaining a lithographic printing plate according to the silver complex diffusion transfer process by subjecting an imaging element to a high intensity short time scanning exposure, in particular an exposure by means of a laser or LED.
The principles of the silver complex diffusion transfer reversal process, hereinafter called DTR-process, have been described e.g. in U.S. Pat. No. 2,352,014 and the book "Photographic Silver Halide Diffusion Processes" by Andr e Rott and Edith Weyde--The Focal Press--London and New York, (1972).
According to the DTR process, a silver complex salt is image-wise transferred by diffusion from the image-wise exposed silver halide emulsion layer into the image receiving layer, where it is converted to a silver image usually in the presence of physical development nuclei. For this purpose, the image-wise exposed silver halide emulsion layer is developed in the presence of a developing agent and non-developed silver halide is converted by means of a silver halide complexing agent into a soluble silver complex salt while in contact with an image receiving layer.
A DTR-image bearing material can be used as a planographic printing plate wherein the DTR-silver image areas form the water-repellent ink-receptive areas on a water-receptive ink-repellent surface. The DTR-image can be formed in the image-receiving layer of a sheet or web material which is a separate element with respect to the photographic silver halide emulsion material (a so-called two-sheet DTR element) disclosed in e.g. DE-A-2.346.378 or in the image-receiving layer of a so-called single-support-element, also called mono-sheet element, which contains at least one photographic silver halide emulsion layer integral with an image-receiving layer in water permeable relationship therewith. It is the latter mono-sheet version which is preferred for the preparation of offset printing plates by the DTR method.
Typically a mono-sheet DTR material comprises on one side of the support a silver halide emulsion layer and a surface layer of physical development nuclei and on the side of the support opposite to the side of the support containing the silver halide emulsion layer one or more backing layers. Typical supports used for such commercial mono-sheet DTR materials are paper or polyester film. The mono-sheet DTR-materials are available in different thicknesses so that one can select a particular material in accordance with the particular demands for the printing job.
In order to prepare a printing plate from such a mono-sheet DTR material, the material can be image-wise exposed by means of a contact exposure or camera-exposure. However, according to more recent developments, the mono-sheet DTR material can be subjected to a high intensity short time scanning exposure such as e.g. a laser or LED exposure. This latter method offers the advantage that full pages including text and images can be composed on a computer which can then send the digital page information to a so-called image-setter including e.g. a laser to expose the monosheet DTR material.
Image-setters are well-known devices in the graphics arts field and are commonly used to expose phototypesetting materials such as photographic films which are subsequently used in a contact exposure for making a printing plate from low sensitive photopolymer plates or diazo plates.
When using such image-setters for making a printing plate from a monosheet DTR material, it was observed in a number of instances that the tone reproduction on the paper copies varied from one plate thickness to another and changed more within the same printing run from one plate to another. This effect was more pronounced when using high screen rulings or when using frequency modulation screening techniques for screening a continuous tone original.